Now you must know jpg or jpeg image right. Well who doesn't. You can use it for fun purposes but avoid to use it in our publication quality documents, as it comes with image compression, so it won't present the best of your work.
Try to use vector image instead or at least save your image to png. The next question is, “what type of image that I should look for or save to?”
You can look for and save your image to pdf, eps, svg. Just one problem, not many wordprocessor recognize non-raster image (except for pdf). Therefore, I usually save my image to png's. This format has relatively higher quality than other raster formats. Both types are well known with all major wordprocessor.
Then, What is the difference of raster and vector image, you can google it or just visit 99designs.
You can compare the quality of "png" image to other rasters. The upper left one is png image, while the rest are jpg, bmp, tif.